Method for concentrating an acrylamide aqueous solution

ABSTRACT

A method for economically concentrating an acrylamide aqueous solution by catalytic hydration without substantial deterioration in the product. According to the method, an acrylamide aqueous solution obtained from catalytic hydration, is concentrated by distillation while maintaining the solution in good contact with at least 0.1 mole. or more of air per mole. of water distilled from the acrylamide aqueous solution.

United States Patent 1 Asano et a].

[ METHOD FOR CONCENTRATING AN ACRYLAMIDE AQUEOUS SOLUTION [73] Assignee:Mitsui Toatsu Chemicals Inc.,

Tokyo, Japan 22 Filed: Dec. 4, i972 2| Appl. No.: 311,830

[30] Foreign Application Priority Data Dec. 6, i971 Japan i. 4697895[52] US. Cl. 260/561 N [5i] Int. Cl. C07C 103/08 [58] Field of Search260/56l N [56] References Cited UNITED STATES PATENTS 2,753,375 7/1956Webb et all i. 260/56] N NOV. 4, 1975 3,257,454 6/1966 Heckle 260/56l N3,274,245 9/1966 Bobsein et al. 260/56l N 3,674,848 7/l972 Schoenbrunnet al 260/561 N X 3,767,706 lO/l973 Habermann et all .5 260/56! NFOREIGN PATENTS OR APPLICATIONS 233,656 5/l969 U4S.SVR i. 260/56l NPrimary Examiner-Lewis Gotts Assistant Examiner-Ethel G. Love [57}ABSTRACT 9 Claims, 1 Drawing Figure US. Patent Nov. 4, 1975 3,917,693

METHOD FOR CONCENTRATING AN ACRYLAMIDE AQUEOUS SOLUTION FIELD OF THEINVENTION The present invention relates to a process for concentratingan acrylamide aqueous solution and producing acrylamide crystals of lowacrylamide polymeric content.

BACKGROUND OF THE INVENTION Several processes have been reportedheretofore for producing acrylamide by means of catalytic hydration.Typical of such processes are those disclosed in U.S. Pat. No.3,381,034, in which a cuprous ion is employed; in U.S. Pat. No.3,597,481, in which an oxide of silver, zinc or cadmium and an oxide ofchromium are employed; in U.S. Pat. No. 3,631,104, in which copperoxide, copper-chromium oxide, copper-molybdenum oxide or a coppercatalyst prepared by reducing any one of such oxides are employed; inU.S. Pat. No. 3,674,848, in which a H3 or II-B group metallic salt ofacid cation exchange resin is employed; and in U.S. Pat. No. 3,673,250,in which a homogeneous catalyst consisting of organic phosphines or thelike and transition metallic compounds are employed. Another exampleinvolves a process which is carried out by use of Raney copper, Ullmanncopper, reduced copper or a catalyst made of substantially any one metalselected from a class including silver, gold and copper with a carrier(US. Patent application, Ser. No. 56,967, filed on July 21, 1970 andowned by the assignee of the present application). To date, however, noprocess suitable for industrial use is known for effectivelyconcentrating an acrylamide aqueous solution by evaporating the watercontent while controlling the acrylamide polymer content in theacrylamide crystal or in the concentrated acrylamide aqueous solutionproduct to less than 0.2 weight in terms of butanol-insoluble residue.

In connection with the storage of such a vinyl monomer as liquid acrylicalkyl ester or the like, it is known that the vinyl monomer can beprevented from polymerizing by saturating it with air and/or oxygen.Howeve in a process wherein an acrylamide aqueous solution isconcentrated by distillation at a high temperature, polymerization couldoccur more readily. Thus, it may be impossible to maintain theacrylamide polymer content in the resulting acrylamide crystals orconcentrated acrylamide aqueous solution at a level less than 0.2 weightmerely by saturating the solution to be distilled with air and/oroxgyen, or by applying the method of the conventional sulfuric acidprocess for producing acrylamide.

It may be clear to those skilled in the art that the technique forlimiting the acrylamide polymer content to a level less than 0.2 weightis extremely important when acrylamide crystals or an acrylamide-richaqueous solution are used in the form of acrylamide monomer, forexample, as a paper reinforcing agent.

OBJECTS OF THE INVENTION The main object of this invention is,therefore, to provide an industrially acceptable process for producingan acrylamide crystal or an acrylamide-rich aque ous solution with onlya small acrylamide polymer content.

Another object of this invention is to provide an improved andeconomical process for concentrating an 2 acrylamide aqueous solution,which process can also be applied to an acrylamide aqueous solutionhaving good polymerizability.

SUMMARY OF THE INVENTION According to the process of this invention, theacrylamide polymer content in the acrylamide crystals or acrylamide-richaqueous solution product can be controlled to be less than 0.2 weight bydistilling an acrylamide aqueous solution obtained from catalytichydration while maintaining the acrylamide aqueous solution in contactwith at least 0.1 mole. of air per mole. of water distilled. Bymaintaining the acrylamide aqueous solution in good contact with 0.1 to30 moles of air, preferably 0.5 to 15 moles. of air, per mole. of waterdistilled, the concentration of an acrylamide aqueous solution becomespossible by distilling the solution at relatively low temperatures andunder normal atmospheric or slightly reduced pressures. As a result, theyield of the acrylamide crystals or acrylamide-rich aqueous solution canbe increased quite ecnomically.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying FIG. 1 is a flowsheet representing a typical system which may be used for carrying outthe process according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION Unlike an acrylamide aqueoussolution prepared by the conventional sulfuric acid process forproducing acrylamide, an acrylamide aqueous solution from catalytichydration contains no significant by-product such as ammonium sulphateand nor appreciable amounts of secondary reaction products, so that itmay be purified by a simple purifying process into a monomer solutionhaving a suitably adjusted concentration and a sufficient purity to betransferred to a subsequent polymerization process. To save intransportation costs or to improve the flexibility in the polymerizationprocessing, the acrylamide aqueous solution may sometimes need to befurther concentrated into a highly concentrated solution or intoacrylamide crystals by crysallization or direct solidification of thehighly concentrated acrylamide aqueous solution. A technically importantproblem which has to be solved in this connection is to inhibit thedeterioration and especially the polymerization of acrylamide. Forexample, when it is preferred that the product contain no polymerizationinhibitor or when the acrylamide is to be solidified directly from theconcentrated solution, the acrylamide crystals and the acrylamidepolymeric content in the concentrated acrylamide aqueous solution needto be controlled to less than 0.2 weight in terms of butanol-insolubleresidue.

With the conventional sulfuric acid process, the ac rylamide and theby-product sulfate are separated from each other by crystallization.More specifically, in such a process the sulfate is usually firstseparated from a ternary system consisting of the acrylamide, a sulfateand water; then, acrylamide is separated by crystalliza tion from theremaining solution which now contains only a small amount of residualsulfate. Thus the coexisting acrylamide polymer, polymerizationinhibitor and other impurities, if any, are left behind in the motherliquor, so that the resulting acrylamide will contain only a limitedamount of these materials. In this process, the polymerizability ofacrylamide itself is also different from that usually encountered in thecatalytic 3 hydration with which this invention is concerned. Thus, onecan readily understand why the concentration technique of the sulfuricacid process cannot be applied to the concentration of the acrylamideaqueous solution obtained from the catalytic hydration.

It may be also readily understood that a concentration process performedunder reduced pressure is to be preferred for concentrating anacrylamide aqueous solution while inhibiting polymerization and otherdeterioration in the product. However, it has been found that aconsiderably reduced pressure, for example 20mml-lg, 30C, must beattained in order to concentrate an acrylamide aqueous solution to 60%without using any polymerization inhibitor. This is not practical forany concentrating operation on an industrial scale, and it is almostimpossible to further promote the concentration for solidification ofthe acrylamide.

It has now been discovered that, when a pure acrylamide aqueous solutionis concentrated to dryness at room temperature and under reducedpressure, it is inevitable for a portion of the acrylamide dissolved inthe solution to be polymerized; on the other hand, if the concentrationprocess for an acrylamide aqueous Solution is performed, for example, atabout 100C under reduced pressure while maintaining the solution incontact with a predetermined amount of air, fused acrylamide whichcontains no polymeric content may be obtained.

While it is known that oxygen or air act as a polymerization inhibitorfor acrylamide, acrylic esters, or the like, it has been a commonpractice heretofore to remove the dissolved oxygen prior topolymerization. On the other hand, the process according to thisinvention possesses unique features and advantages as indicated in thefollowing paragraphs:

1. in order to achieve a satisfactory polymerization inhibiting effect,it is necessary to maintain the acrylamide aqueous solution in goodcontact with air under agitation, for example, by using a greater amountof air than is commonly employed. As a result, the following twodifferent effects can be achieved simultaneously and the polymerizationinhibiting effect can be strikingly improved.

A. The use of a large amount of air results in a good contact betweenthe solution and the air. Since the air also acts as a diluent for thewater vapor generated, practically the same effect can be achieved as ina concentration process under reduced pressure. If air is added to theconcentrated solution in a equimolar amount with the water vaporgenerated at a time when the concentrated solution has a concentrationof 60%, the temperature of the concentrated solution will be 88C, whichis 17 lower than the temperature that the concentrated solution mightotherwise reach, if no air were employed. Consequently, the rate ofpolymerization of the acrylamide is repressed to a remarkable extent.

B. As the temperature falls, the solubility of the oxygen into anacrylamide aqueous solution increases. By using an increasingly largeramount of air, an increas ingly higher partial pressure of the oxygencontained in the gaseous phase, i.e., a gaseous mixture of air and watervapor which are in contact with the acrylamide aqueous solution, will beobtained. This will lead to an increase in the amount of oxygen which isdissolved into the solution.

2. The polymerization inhibiting effect achieved permits theconcentration of an acrylamide aqueous soluapparatus is required fordistillation under reduced pressure, and the ease in concentrating anacrylamide aqueous solution by distillation is considerable.

3. Fused acrylamide may be obtained by nearly com pletely evaporatingthe water content from the acrylamide aqueous solution. When cooled, thefused acrylamide may be crystallized. By applying the process of thisinvention to a highly purified aqueous solution which has been obtainedfrom the catalytic hydration process, highly purified acrylamidecrystals may be obtained directly from a crystallization process whichis greatly simplified compared with any other commonly employed process.

Although acrylamide is a highly reactive compound and enters readilyinto polymerization, carbamyl ethylation or other amide group reactions,no secondary reaction occurs when an acrylamide aqueous solution isconcentrated at a high temperature of C or so, for example, inaccordance with the process of this invention, but crystals having apurity of 99.0% or above may readily be obtained when, for example, thesolidified acrylamide is dried.

4. The process of this invention is most advantageous when it is appliedto an acrylamide aqueous solution. The reason for this lies in thatacrylamide is scarcely distilled due to its low vapor pressure and hencethe recovery of water vapor by condensation is either unnecessary or itcan be effected in a simple manner. In addition, the concentratorrequires no fractionating section for separation of acrylamide fromwater. Thus, the process avoids any possible difficulty which mayotherwise result from a relatively large amount of air passing throughsuch a fractionating section.

It is known that such advantage as described above at (4) cannot befully expected when an unreacted raw material such as acrylonitrile ispresent, or when a third component such as methanol is added for thepurpose of improving solubility, or when the concentration process isapplied to a vinyl polymerization type monomer having a low boilingpoint other than acrylamide, for example, an acrylic ester.

As noted above, when an acrylamide aqueous solution obtained from acatalytic hydration is to be concentrated by distillation, the solutionfirst needs to be treated by means of a usual distillation process forremoving acrylonitrile, if any.

The process of this invention may be carried out in the following fourdifferent manners; that is:

l In the manner disclosed hereafter in Examples 1 to 6, in which anacrylamide aqueous solution is heated while bubbling air thereinto;

2. In the manner disclosed in Example 7, in which an acrylamide aqueoussolution is sprayed into a flow of hot air to separate a concentratedsolution for directly preparing acrylamide crystals;

3. In the manner disclosed in Example 8, in which an acrylamide aqueoussolution is concentrated into a thin liquid film within an air flow; and

4. In the manner disclosed in Example 9 in which a preheated acrylamideaqueous solution is mixed with air and concentrated by distillation.

Of these four variants, two are more economically applicable toindustrial purposes. They are specifically set forth in Example 4 (inwhich a double pipe concentrator is employed and an acrylamide aqueoussolution is permitted to flow in parallel with air) and in Example 9 (inwhich an apparatus is employed with liquid heatmg section and a waterevaporating section as shown in FIG. 1). With either one oftheabove-referred variants, the polymerization inhibiting effect can beachieved in the most simple manner. The so-called spray process, asillustrated in Example 7, is also useful for directly obtainingacrylamide crystals. Furthermore even a process for concentrating anacrylamide aqueous solution by distillation with use of a conventionalpacked tower may be employed.

l'he quantity of air feed may vary depending upon the variant processand the acrylamide concentration if the solution employed. At aconcentration of less than 80 0.1 to times as many moles. of air asthere are moles of distilled water may be employed. At higherconcentrations, a larger quantity of air, for ex- :imple 30 moles of airper mole of distilled water, may be employed. Thus, the preferred airfeed may be in the range of0,5 to 15 moles. per mole. of water to beevaporated. When the moles of air are less than 0.1 times tllC number ofmoles of water to be evaporated, no such effect may be achieved; on thecontrary, when the moles. of air exceed 30 times the moles. of water tobe evaporated, satisfactory polymerization inhibiting ef iect may beachieved but, in such a case, excessively large and uneconomicalequipment may be required.

As mentioned before, by supplying a large quantity of dlf to thedistillation system, an acrylamide aqueous sotution can be concentratedat a relatively high tempera- Lure of 100C or so without accompanyingformation of acrylamide polymer. However, when the concentration iscarried out under reduced pressure, a pressure of at least 200 mmHg isnormally required, because the oxygen dissolved in the acrylamideaqueous solution is reduced in quantity at a pressure of less than 150mmHg. On the other hand, the concentration under a slightly increasedpressure, for example 5 kg/cm (3,678 mmHg) or so, preferably less than 2kg/cm (1,470 mmHg) will incur no specific difficultyv Examples ofacrylamide aqueous solutions to which the process of this invention canbe applied are the acrylamide aqueous solutions obtained from variouscatalytic hydration processes. These processes include those whichemploy such a catalysts as Raney copper, Ulmann copper, reduced copper,copper with a carrier, silver or gold; those where a cuprous ion or anoxide of silver, zinc, cadmium or chromium are used; those in whichcopper oxide, copper-chromium oxide, copper molybdenum oxide or a coppercatalyst obtained by reducing any one of these oxides is used; those inwhich 1-8 or [1-8 group metallic salts of an acid cation exchange resinare used; and those in which a homogeneous catalyst composed of organicphosphines and transition metallic compounds is used. Similar effectsmay be obtained by applying the process of this invention to aneutralized mother liquor which is obtained by removing ammonium sulfatefrom a liquid resulted from the reaction according to the conventionalsulfuric acid process for producing acrylamide, or by applying it to anaqueous solution having dissolved therein acrylamide crystals preparedby the sulfuric acid process.

Actually, there is no limitation in the concentration ofthe aqueoussolution to be employed. Even an aquenus solution having an extremelylow concentration may be concentrated into a so-called fused acrylamidewhich has a concentration of 95% above.

The presence of other solvents in the aqueous solution will have noadverse result; however, the advantages of this invention may sometimesfail to be fully achieved for that reason. Air may be used diluted withnitrogen or, conversely, added to oxygen to such an extent that itincurs substantially no danger.

The invention will now be described in greater details by way ofexamples. In the following examples, the results have been obtained inaccordance with standard definite analytical methods, which will bedescribed. Concentration or purity of the acrylamide have been measuredby bromometry. The measurement of pH was made by a usual pH meter. Thecolor tone was measured by the so-called APHA method adapted formeasuring the concentration of yellowish brown color in a liquid andsuch measurements were made for comparison purpose for solutions havingan equal concentration of about 30%.

The butanol-insoluble residue was measured in the following manner; 7gof pure acrylamide was dissolved into ml of butanol. The undissolvedacrylamide was then separated by filtration and washed with a smallamount of butanol. After drying at 100C for 2 hours, the residualacrylamide was weighed. Most of the butanol-insoluble residue consistedof acrylamide polymer, but other components such as inorganic salts, ifany, were also added in the measurement.

The determination of the salt content was made in the following manner:The sample solution (when the sample was in the form of crystal, it wasdissolved in water to prepare an aqueous solution having a concentrationof about 30%) was first treated with a cation exchange resin of the SO Htype to liberate the acid, which was then titrated with caustic sodausing bromothymol blue as an indicator. The value of the salt contentnormally represented the quantity of such salts as acrylic ammonium andthe like, but the other inorganic salts, if any, were also added in themeasurement.

EXAMPLE 1 75 parts by weight/hr of Raney copper catalyst and 785 partsby weight/hr of water were continuously sup plied in the form of aslurry to a reactor containing 500 parts by wt. of Raney copper. At thesame time, acrylonitrile was supplied to the reactor at a rate of 383parts by wt, per hour. The mixture was retained within the reactor for2.5 hours for reaction at C. Then the reaction solution which containedalmost no catalyst was removed from the top of the reactor, while acorresponding feed portion of the catalyst slurry was withdrawn at thebottom of the reactor. The composition of the liquid obtained byremoving the catalyst from the reaction solution and the catalyst slurrywas as follows:

Acrylonitrile 15 weight Acrylamide 20 weight Water 65 weight (Conversionratio, 50%) 7 separated by means of filtration. And deionization iscarried out using a column packed with a sulfonic acid type cationexchange resin by means of a dilute acid. Therefor, caustic soda isadded to make its pH to nearly 6.5.

3,000g of such 30% acrylamide aqueous solution were then disposed in aflask provided with an agitator and immersed in a l 10C bath; theacrylamide aqueous solution was concentrated under normal pressure whilebubbling air of ambient temperature at arate of 570 l/hr. (normalcondition). During the concentration, the temperature of the solutionwas about 85C and the quantity of the distilled water was about 300g/hr. in terms of moles, the air feed and the distilled water were 25.4g-mole/hr. and 16.7 g-mole./hr. respectively; their molar ratio was 1.5.

After hours, when the concentration of the solution appeared to havereached 60%, the concentration process was ceased. Then, theconcentrated solution was analyzed together with the originalunconcentrated solution. The results are set forth in the followingtable:

(vs. acrylamide Salt Content (milliequiva- 6.3 7.2 lent/kg acrylamide)For purpose of comparison, 3,000g of an identical 30% aqueous solutionwere disposed in a flask immersed in a l C bath; and the solution wasconcentrated under reduced pressure of about 300 mmHg (absolutepressure) while bubbling air of ambient temperature into the solutionthrough the tip of capillary at a rate of Uhr (normal condition). Duringthe concentration, the temperature of the aqueous solution was about 85Cand the quantity of the distilled water was about 400 g/hr. In terms ofmoles, the air feed and the distilled water were 0.9 g-mole./hr. and22.2 g mole./hr., respectively; their molar ratio was 0.04. After about4 hours, when the concentration of the solution appeared to have reachedabout 60%, the concentration process was ceased and the concentratedsolution was analyzed. The results were as follows:

Concentration of Acrylamide (11) 59.0 pH 7.05 Color Tone (APHA) 20 (foraqueous solution) Butanol-lnsoluhle Residue 0.24

(vs. acrylamide 9ft) It will be seen from the preceding that the polymercontent in the concentrated solution is relatively increased.

EXAMPLE 2 An experiment similar to that disclosed in Example 1 wasperformed under a reduced pressure of 230 mmHg. During theconcentration, the temperature of the acrylamide aqueous solution wasabout 60C and the quantity of the water distilled was about 600 g/hr. interms of moles, the air feed and the distilled water 8 were 25.4g-mole./hr. and 33.3 g-mole./hr., respectively; their molar ratio was0.1. After 2 hours, when a concentration of about 60% appeared to havebeen reached, the concentration process was ceased and the concentratedsolution was analyzed. The results were as follows:

Concentration of Acrylamide (Z (v) 62.1 pH 6.8 Color Tone (APHA) 10 (for30% aqueous solution) Butanol-lnsoluble Residue 0.05 (vs. acrylamideSalt Content 6.5

(milliequivalent/kg acrylamide) Almost no deterioration was observed inthe concentrated solution except that the butanol-insoluble residue wasincreased slightly compared with the original aqueous solution.

EXAMPLE 3 300g ofa 30% aqueous solution identical to that used inExample 1 were disposed in a flask having an agita tor and immersed in a100C bath and the concentration was carried out at normal temperaturewhile bubbling air of ambient temperature at a rate of 600 l/hr. (normalcondition). The temperature of the acrylamide aqueous solution was aboutC at an early stage of the concentration process, and then thetemperature of the solution rose gradually until it reached about Cafter minutes, when the evaporation of water was scarcely observed. Thedistilled water was 210g in to tal. The rate of the water, although itwas gradually lowered as the process proceeded, was 126 g/hr. inaverage. Thus, in terms of moles, the air supplied and the waterdistilled were 26.8 g-mole./hr. and 9.5 gmole./hr., respectively; theirmolar ratio was 2.8. Under continuous agitation and air supply, theconcentrated solution was cooled for solidification. Thebutanol-insoluble residue of solidified acrylamide was 0.03%, and thecolor tone (APHA) of the 30% acrylamide aqueous solution prepared bydissolving the resulted acrylamide in water was 20.

For purpose of comparison, 100g of an identical 30% aqueous solutionwere disposed in a flask immersed in a 30C bath and the concentrationwas carried out under a reduced pressure of 10 mmHg (absolute pressure).After about 15 minutes, when crystallization was observed, theconcentration process was ceased and the pasty content consisting of acrystallized and vis cous polymer was taken out of the flask. Identicalexperiments were repeated only to attain the same result. Similarresults were obtained from an identical experiment which was conductedby using a 30% aqueous solution prepared by dissolving in water acommercially available crystallized acrylamide prepared by the sulfuricacid process.

EXAMPLE 4 At the bottom of the inner pipe of a double pipe concentrator(the concentrator including a l-inch SUS-27 steel pipe and a 2-inchSUS-27 steel pipe), 540 l/hr. of air (normal STP condition) and 2,120g/hr. of 38% acrylamide aqueous solution prepared as described inExample l were supplied into the concentrator, while permitting 1,470g/hr. of concentrated solution, 740 g/hr. of distilled water and air toflow out of the concentrator at the top of the inner pipe. Whilesupplying water vapor as a heat source through the outer pipe at anormal pressure, the concentration was carried out at about 95C under anormal pressure. In such a process, the residence time of the solutionin the concentrator seemed to be about 10 minutes. (The above-described5 concentration process will simply be expressed hereun der as thefirst-stage concentration).

Subsequently, l,440 g/hr. of the concentrated solution resulted from thefirst-stage concentration and 1,350 l/hr. of air (normal STP condition)were supplied into the same concentrator to carry out another concentration at about 92C under ambient temperature. As a result, 840g/hr. of further concentrated solution and 600 g/hr. of distilled waterwere obtained. (Such a concentration process will simply be calledhereunder as the second-stage concentration.) In order to preventcrystallization, the concentrated solution resulted from thesecond-stage concentration was placed in a reservoir maintained at 60C.This reserve solution was then poured into SUS-27 steel dishes at aninterval of minutes to let the solution solidify into flake-like solidsby air-cooling. Using a rotary dryer, the resulted flakes were dried for2 hours at 60C to yield dried crystals. In the firstand second-stageconcentrations, the moles. of the air feed and the distilled water wereas set forth in the following table:

TABLE II First-Stage Second-Stage Concentration Concentration AirDistilled Air Distilled Feed Water Feed Water Flow Rate in Moles. 24.041.0 60.3 33.3 (g-moIeJhr.) Molar Ratio 0.6 L8

With respect to the liquids and solids obtained from each stage ofconcentration, the analytical results were as follows:

It will now be clear to those skilled in the art that as theconcentration and drying operations proceed, the values of color tone,butanol-insoluble residue and salt content are increased slightly butnot to such an extent as to reduce the value of the resulted product asa commodity.

g of dried crystals were then dissolved in 450g of water within a flaskhaving an agitator and immersed in a 40C bath. Dissolved oxygen wasremoved by bubbling nitrogen into the solution. When the temperature ofthe solution reached 40C, 2 ml. of 15% ammonium persulfate aqueoussolution and 4 ml. of 1% sodium hydrosulfate aqueous solution were addedsimultaneously under agitation. After an induction period of seconds,when the temperature rise indicative of the initiation of polymerizationwas observed, the agitation was ceased temporarily. After that, when thetemperature began to fall, agitation was resumed and continued for 0another 2 hours. The viscosity of the resulted solution at a temperatureof 25C was 102 poise (Brookfield Viscometer).

An identical polymerization reaction was carried out with a 10% aqueoussolution prepared from the same original solution. In this case, theinduction period was seconds and the viscosity of the polymerizedsolution was poise. This shows that almost no product deteriorationresulted from the concentration and crystallization operations.

After about 2 days of operation, an inspection of the interior of theconcentrator was made. No deposition of polymers was found on theheat-transfer surface or any other surface areas. This means that nodifficulty will be encountered during a long period of operation.

EXAMPLE 5 An experiment similar to the first-stage concentrationdisclosed in Example 4 was carried out with reduced air feed of l/hr.(normal STP condition) and by using water vapor controlled in such amanner that the resulting solution had a concentration similar to thatof the solution obtained from Example 4. As a result, the temperature ofthe solution in the concentrator reached 102C, and 700g of distilledwater and 1,430g of 56% concentrated solution were obtained per hour. Inthis experiment, the air feed and the distilled water were, in terms ofmoles. 5.8 g-mole./hr. and 38.9 g-mole./hr., respectively; their molarratio was 0.15.

The butanol-insoluble residue contained in the concentrated solution was0.14% (vs. acrylamide Although this percentage is slightly higher than0.01% (vs. acrylamide as for the solution from the first stageconcentration of Example 4, its quantity is so insignificant as to incursubstantially no practical disadvantage.

EXAMPLE 6 Using commercially available acrylamide crystals manufacturedby the sulfuric acid process, a 40% acrylamide aqueous solution wasprepared. Following the operation in Example 4, dried crystals wereobtained from the solution. The following table represents in comparisonthe analytical results ofthe original crystals and the resulted drycrystals.

TABLE IV Original Resultcd Dry Crystals Crystals Purity of Acrylamide98.0 98.3 Water Content (96) 0.8 0.3 Color Tone (APHA) 30 40 (for 30%aqueous solution) Butanol-lnsoluble Residue 0.13 0.20 (vs. AcrylamideSalt Content 350 39.7 (milliequivalent/kg acrylamide) [n the resulteddry crystals, color tone, butanol-insoluble residue and salt content areincreased slightly, but not to such an extent as to reduce the value ofthe resulted crystals as a commodity.

EXAMPLE 7 An experiment was made using a spray dryer provided with acylindrical container body 2m in diameter and 2.5m high, and soconstructed as to have a conical bottom and a spray means mounted ontop. A 60% concentrated acrylamide aqueous solution obtained from thefirst-stage concentration in Example 4 was supplied into the spray dryerat ambient temperature at a rate of 12 l/hr. and sprayed. At the sametime, 120C air was supplied into the spray dryer at the top thereof at arate of IO m /rnin., while discharging crystallized acrylamide togetherwith air (cooled to 65C) from the bottom of the spray dryer. Dischargedacrylamide crystals were collected in a bag filter.

The water content and the butanol-insoluble residue were 0.7% and 0.03%,respectively.

In this experiment, the distilled water was about 7 kg/hr. The distilledwater and air feed were in terms of moles, 0.39 g-mole./hr. and 0.45g-mole./hr, respectively; their molar ratio was 1.1.

EXAMPLE 8 A double pipe assembly 0.8m long and consisting of a SUS-27steel pipe of l-inch diameter and a SUS-27 steel pipe of 2-inch diameterwas employed as a wettedwall concentrator. Water vapor was introducedinto the outer pipe as a heat source at ambient temperature; air wassupplied into the inner pipe at the bottom thereof at a rate of 550l/hr. (normal STP condition), and a 30% aqueous solution as in Example 1was supplied into the inner pipe at the top thereof so that it floweddown along the inner wall surface of the inner pipe in the form of auniform liquid film. The distilled water was discharged from the top ofthe concentrator at a rate of 240 g/hr. together with air, while theconcentrated solution was collected at a rate of 430 g/hr. at

the bottom of the concentrator. In terms of moles, the

air feed and the distilled water were 24.5 g-mole./hr.

and 13.3 g-mole./hr., respectively; their molar ratio was 1.8. Theconcentration of the resulted solution was measured to be 48%, thebutanol-insoluble residue could not be detected as it was the case inthe original 50 30% aqueous solution.

EXAMPLE 9 A concentration as in Example 4 was carried out using asecond-stage concentrator, an apparatus provided with a liquid-heatingsection and a waterevaporating section as shown schematically in Flg. l.A liquid heater 4 comprises a jacket encircling a fi-inch SUS-27 steelpipe to define an annular space between the jacket and the pipe, so thatsteam can be supplied into the space at ambient temperature. A packedtower 5 comprises a 2-inch dimeter, 80cm long SUS steel pipe, in whichporcelain inch Raschig rings are placed.

An aqueous solution obtained from the process of Example 4 was firstdisposed in a first-stage concentrated solution reservoir 1. By use of afeed pump 2, the solution was forced into the packed tower 5 afterpassing through the liquid heater 4 at a rate of 1,370 g/hr. Air whichhad been preheated to 100C by means of an air-heater 6 was suppliedsimultaneously into the packed tower 5 at a rate of 5.1 N m lhr. Bothair and solution were brought into good contact with each other whileflowing down within the packed tower 5, so that a concentrated solutionhaving a temperature of about C and air containing some water wereobtained in the packed tower 5. The concentrated solution and the airwere then introduced into a first liquidgas separator 7 and separatedtherein from each other. The air having water therein was then forced topass through a condenser 8 to thereby separate the water from the air.The concentrated solution, on the other hand, was heated in the liquidheater 4 to C together with the solution coming from the feed pump 2 ata rate of 12.5 kg/hr., and returned to the packed tower 5 by means of apump 3. At the same time, a portion of the concentrated solution wascontinuously removed to a second concentrated solution reservoir 10which was maintained at 60C. The concentration of such a solution was92%.

In terms of moles, the air feed and the distilled water were 228g-mole./hr. and 250 g-mole./hr., respectively, their molar ratio was9.1.

As in Example 4, the concentrated solution was dried into flakes andthen into dry crystals. The following table represents in comparison theanalytical results of the original solution, the concentrated solutionfrom the first-stage concentration, the flakes and the dry crystals.

In this experiment, the color tone and the butanolinsoluble residue wereincreased slightly as the concentration or drying operations proceeded,but not to such Residue (vs. Acrylamide detected an extent as to reducethe value of the final product as a commodity.

After continuing such a concentration operation for 3 days, aninspection was made of the interior of the apparatus. No deposition wasfound either in the liquidheating section or in the packed tower 5. Thismeans that the illustrated apparatus is capable of withstanding acontinuous operation for an extended period of time.

EXAMPLE Tests were carried out for the acrylamide aqueous solutionsprepared by various catalytic hydration processes. In the cases where itwas required to do so, each solution was tested after removal ofunreacted acrylonitrite residue contained therein. Each of the samplesoiutions was prepared by a definite catalytic hydration process, asdescribed herebelow.

SAMPLE 1 ln a 30mm diameter and 300mm long reaction tube were charged390g (or 220ml in volume) of cupric xide pellets (manufactured by NikkiChemical Company ltd. The pellets were then reduced at "P ()--27OC withhydrogen gas and nitrogen gas passing through the reaction tube at arate of 200 ml/min. and 400 ml/min, respectively, to thereby prepare areduced copper catalyst. From the solution in weight of the pellets, itwas found that the reduction ratio of the resulted catalyst was 98%.Then, acrylonitrile and water were continuously supplied to the reactiontube :tt a rate of 140 g/hrv and 690 g/hr., respectively to arry outreaction at 120C. The reaction solution was force-circulate within thereaction tube at a rate of 40 /hr. to thereby promote the preparation ofan acrylamide aqueous solution. The conversion ratio of acrylonitrileinto acrylamide was 70%.

SAMPLE 2 A reduced copper-chromium catalyst was prepared by a reducingoperation which was similar to that employed for preparing the reducedcopper catalyst except that 470g of copper-chromium pellets(manufactured by Nikki Chemical Co., Ltd.) were used. Then, using such areduced coppenchromium catalyst, a catalytic hydration was conductedunder conditions of Sample 1. Accordingly, an aqueous solution ofacrylamide was prepared at an almost equal conversion ratio.

SAMPLE 3 In a ll reaction tank (made ofSUS-27 stainless steel) providedwith a catalyst separator and an agitator, were disposed 250g of copperpowder. Then, acrylonitrile and water (dissolving therein onesixty-ninth parts by weight of cupric chloride) were continuouslysupplied to the reaction tank at a rate of I40 g/hr. and 690 g/hr.respectively and reacted at 120C. The conversion ratio of acrylonitrileto acrylamide was 14%.

SAMPLE 4 in the reaction tank of Sample 4, were disposed 250g of aheterogeneous catalyst consisting of 90 weight silver oxide and 10weight chromium oxide. Then, acrylonitrile and water were continuouslysupplied to the reaction tank at a rate of 140 g/hr. and 690 g/hr.respectively and reacted at lC. The conversion ratio was 32%.

SAMPLE 5 Zinc resinate was prepared by treating a commercially availablesodium type amberlite [RC-50 (registered trademark) with 5% zincchloride aqueous solution. In the same reaction tank as in Sample 3,200g of acrylonitrile, 300g of water and g of zinc resinate were reactedfor 4 hours at C, in the presence of a small quantity of an oxidationinhibitor. The conversion ratio of acrylonitrile to acrylamide was l l%.

The acrylamide aqueous solutions prepared from the above-describedvarious processes were purified in accordance with the process ofExample 1, and concentrated in accordance with the process of Example 2.The results of such concentration operations were as follows:

As far as pH, color tone, butanoLinsoluble residue and salt content areconcerned, the relationship between each of the acrylamide concentratedaqueous solutions and each corresponding unconcentrated acrylamideaqueous solution was found to be almost equal to that in the acrylamideaqueous solution prepared by using a Raney copper catalyst.

What is claimed is:

1. Method for concentrating acrylamide aqueous solutions comprisingconcentrating the acrylamide aqueous solution by evaporating the watercontent thereof while maintaining the said solution in contact with from0.1 to 30 moles of air per mole of water being evaporated at atemperature of between 60C and l00C and a pressure of between 3,678 and200 mm. Hg.

2. Method according to claim 1, wherein the acrylamide aqueous solutionis concentrated while maintaining the solution in contact with 0.5 to 15moles. of air per mole. of water being evaporated.

3. Method according to claim 1, wherein the acrylamide aqueous solutionis the product of a catalytic hydration and the solution is concentratedafter removal of unreacted acrylonitrile residue contained therein.

4. Method according to claim 3, wherein said catalytic hydration employsa catalyst selected from the group consisting of Raney copper, reducedcopper, reduced-copper chromium, copper powder-cupric chloride, silveroxide-chromium oxide and zinc salts of acid cation exchange resins.

5. Method for concentrating an acrylamide aqueous solution according toclaim 2, wherein said acrylamide aqueous solution is the product of aRaney copper-catalyzed hydration and wherein said solution is concen- 16air to separate a concentrated solution for obtaining acrylamidecrystals.

8. The method according to claim 1, wherein the acrylamide aqueoussolution is concentrated into a thin liquid film within an air flow.

9. The method according to claim 1, wherein the acrylamide aqueoussolution is first preheated, then mixed with air and, finally,concentrated by evaporation.

1. METHOD FOR CONCENTRATING ACRYLAMIDE AQUEOUS SOLUTIONS COMPRISINGCONCENTRATING THE ACRYLAMIDE AQUEOUS SOLUTION BY EVAPORATING THE WATERCONTENT THEREOF WHILE MAINTAINING THE SAID SOLUTION IN CONTACT WITH FROM0.1 TO 30 MOLES OF AIR PER MOLE OF WATER BEING EVAPORATED AT ATEMPERATURE OF BETWEEN 60*C AND 100*C AND A PRESURE OF BETWEEN 3,678 AND200 MM. HG.
 2. Method according to claim 1, wherein the acrylamideaqueous solution is concentrated while maintaining the solution incontact with 0.5 to 15 moles. of air per mole. of water beingevaporated.
 3. Method according to claim 1, wherein the acrylamideaqueous solution is the product of a catalytic hydration and thesolution is concentrated after removal of unreacted acrylonitrileresidue contained therein.
 4. Method according to claim 3, wherein saidcatalytic hydration employs a catalyst selected from the groupconsisting of Raney copper, reduced copper, reduced-copper chromium,copper powder-cupric chloride, silver oxide-chromium oxide and zincsalts of acid cation exchange resins.
 5. Method for concentrating anacrylamide aqueous solution according to claim 2, wherein saidacrylamide aqueous solution is the product of a Raney copper-catalyzedhydration and wherein said solution is concentrated under a pressure inthe range of 3, 678 to 200 mmHg while maintaining said solution incontact with 0.5 to 15 moles. of air per mole. of water beingevaporated.
 6. The method according to claim 1, wherein the acrylamideaqueous solution is heated while bubbling air thereinto.
 7. The methodaccording to claim 1, wherein the acrylamide aqueous solution is sprayedinto a flow of hot air to separate a concentrated solution for obtainingacrylamide crystals.
 8. The method according to claim 1, wherein theacrylamide aqueous solution is concentrated into a thin liquid filmwithin an air flow.
 9. The method according to claim 1, wherein theacrylamide aqueous solution is first preheated, then mixed with air and,finally, concentrated by evaporation.